Conventional electronic data memories, for example dynamic random access memory (DRAM) or flash RAM, are increasingly running into limits when they are supposed to satisfy modern requirements. Conventional concepts for electronically storing data, as are also used in DRAM or flash RAM, store information units in capacitors, in which case a charged or uncharged state of a capacitor may represent, for instance, the two logic states “1” or “0”.
In the case of DRAM, the capacitors are of extremely small design, in order to achieve high information density and integration, and therefore require the stored information content to be continuously refreshed. In addition to additional memory controllers for the purpose of refreshing, this also requires a considerable amount of power. On the other hand, flash RAM retains the information content stored in it even without power being supplied, but individual flash RAM memory cells are relatively large and require a high voltage for writing information. Modern electronic data memories must therefore be capable of combining high information density, fast access, and non-volatility. In this context, non-volatility denotes the property of an electronic data memory of being able to store information reliably for a considerably long period of time without the need of external power supply.
The requirements in terms of integration density and non-volatility become apparent particularly in the case of mobile applications, since, in that case, available space is limited and batteries—serving as the power supply—may provide only a limited amount of power and also only a limited voltage. In order to combine non-volatility with fast access time and high integration, alternatives for DRAM or flash RAM are subject to intensive scientific and industrial research. In this case, the so-called resistive electronic memories—inter alia—represent a promising concept.
In addition to solid electrolytes, phase transition materials and other special materials, a high-resistive and a low-resistive state may also be imposed on transition metal oxide layers, which, in this way, may serve as a reliable and stable resistive memory cell. By way of example, a logic state “1” may thus be assigned to a low-resistive state and a logic state “0” may be assigned to a high-resistive state. Furthermore, such layers also allow a distinction to be made between a plurality of resistive states, with the result that a plurality of distinguishable logic states may be kept reliably in a single cell, which is also referred to as multibit capability.
The process of storing information in a transition metal oxide (TMO) layer is based on the principle that a low-resistive filament can be formed in a TMO by means of local heating. Local heating may be generated by a current through the TMO which initially has a high resistance. As a result, the filament short-circuits the otherwise high-resistive TMO and thus considerably changes the effective electrical resistance. By means of application of a sufficiently low volatage the resistive and thus the logic state of the memory cell may be determined via measuring a resulting current. An existing filament may be interrupted again using a sufficiently high current and the TMO memory cell thus reverts to a high-resistive state. This process is reversible and has also already been shown at technically relevant repetition rates in the range of 106. In general, a TMO memory cell may be formed from a lower electrode, an upper electrode and a TMO layer arranged in between said electrodes. The minimum size in this case of such a TMO memory cell is primarily given by lithographic restrictions as far as the patterning of the electrodes is concerned.
An individual filament which considerably reduces the electrical resistance of a TMO memory cell often has a much smaller cross section than the minimum contact area of the electrodes which can be achieved using modern lithography and patterning techniques. During an initial programming step, a plurality of filaments begin to form until a first continuous filament short-circuits the upper and lower electrodes. This also terminates the further formation of the residual filaments which do not continue to grow after the short circuit caused by the first contiguous filament. However, the formation of these residual filaments is unnecessary as far as the programming is concerned, since only one individual filament suffices to reliably define the resistive state of the TMO memory cell. The spatial extent and the size of the cross-sectional area of the at least one continuous filament is not subject to any control either and hence the resistivity of a considerable volume of the TMO is changed unnecessarily. Nevertheless, the formation of the residual filaments and the unnecessary altering of volume requires current heating and hence power is consumed unnecessarily. However, it is desired, to keep the power required to write to, and read from, modern electronic data memories as low as possible.